ci: free runner disk for add-on jobs and fix the Memory delete spec

The add-on jobs stop in prepare.sh when less than 20 GiB is free, and
the hosted runner image no longer leaves that much. Add a small script
that removes the unused preinstalled toolchains in parallel and prints
df before and after, and run it first in both jobs.

The Memory delete spec expected one status element. The page live
region and the toast container both carry role=status, so two match.
Check the page live region on its own and the toast as the second.

Assisted-by: Claude:claude-sonnet-5-5

scripts/ci/free-runner-disk.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+# Frees disk space on a GitHub-hosted ubuntu runner before a job installs its
+# own toolchain. It removes preinstalled toolchains that the add-on jobs never
+# use. It keeps Go, Node.js, Python 3 from the OS and everything under
+# /usr/local/bin, because setup steps and the job rely on them. The deletes run
+# in parallel; the script prints df before and after.
+set -u
+echo "== disk before"
+df -h / /mnt 2>/dev/null || df -h /
+paths="
+/usr/share/dotnet
+/usr/local/lib/android
+/opt/ghc
+/usr/local/.ghcup
+/usr/local/share/powershell
+/usr/local/share/chromium
+/opt/microsoft
+/opt/az
+/usr/share/swift
+/usr/local/julia*
+/opt/hostedtoolcache/CodeQL
+/opt/hostedtoolcache/Python
+/opt/hostedtoolcache/PyPy
+/opt/hostedtoolcache/Ruby
+/opt/hostedtoolcache/Java_*
+"
+for p in $paths; do
+    # Unquoted on purpose: some entries are globs.
+    for match in $p; do
+        [ -e "$match" ] || continue
+        sudo rm -rf "$match" &
+    done
+done
+sudo docker image prune --all --force >/dev/null 2>&1 &
+wait
+echo "== disk after"
+df -h / /mnt 2>/dev/null || df -h /